1. Field of the Invention
The present invention relates to a vacuum coating forming device for forming on a substrate a thin-film coating made of an electrically insulating material.
2. Description of the Prior Art
As a vacuum coating forming device for forming on a substrate a thin-film coating of an electrically conductive material or an electrically insulating material, there have conventionally been known an ion plating apparatus as shown in FIG. 20 and a plasma enhanced CVD apparatus as shown in FIG. 21.
In the ion plating apparatus shown in FIG. 20, plasma is generated in a vacuum chamber 12 by a pressure gradient type plasma gun (hereinafter, referred to as plasma gun) 11, whereby a thin-film coating is formed on a substrate 13 placed in the vacuum chamber 12 by vapor deposition.
In more detail, the plasma gun 11 comprises an annular cathode 15 connected to the negative side of discharge power supply 14 and, annular first intermediate electrode 16 and second intermediate electrode 17 connected, respectively, to the positive side of the discharge power supply 14 via resistors, receiving the supply of discharge gas from the cathode 15 side, and setting the discharge gas in plasma state so as to deliver the discharge gas in plasma state from the second intermediate electrode 17 toward an inside of the vacuum chamber 12. The vacuum chamber 12 is connected to an unshown vacuum pump and the interior of the vacuum chamber 12 is held at a predetermined depressurized state. Furthermore, a converging coil 18 is provided outside a short-tube portion 12A of the vacuum chamber 12 projecting toward the second intermediate electrode 17 so as to surround the short-tube portion 12A. At a lower portion in the vacuum chamber 12, a hearth 19 is disposed which is connected to the positive side of the discharge power supply 14 and made of an electrically conductive material. A recession is formed on the hearth 19, in which an electrically conductive or insulating deposition material 20 used as a material of the thin-film coating is contained. In addition, a magnet 21 for the hearth 19 is provided within the hearth 19.
By this arrangement, a plasma beam 22 is formed so as to be directed from the second intermediate electrode 17 toward the deposition material 20, making the deposition material 20 evaporated and deposited onto the lower surface of the substrate 13, whereby a thin-film coating is formed. Meanwhile, the converging coil 18 functions to reduce a cross section of the plasma beam 22, while the magnet 21 for the hearth 19 functions to lead the plasma beam 22 to the hearth 19.
On the other hand, in the plasma enhanced CVD apparatus shown in FIG. 21, an anode 31 connected to the positive side of the discharge power supply 14, a magnet 32 for the anode 31 disposed behind the anode 31, i.e., on a side opposite to the plasma gun 11 side, and a material gas supply tube 33 are provided instead of the hearth 19, the deposition material 20 and the magnet 21 of the foregoing apparatus. Other constructions of the plasma enhanced CVD apparatus are substantially identical with the foregoing apparatus.
By this arrangement, material gas and reactive gas are supplied into the vacuum chamber 12 through the material gas supply tube 33, and these gases are separated from and combined with each other by plasma so as to be deposited onto the substrate 13, by which a thin-film coating is formed on the substrate 13.
In case of formation of a thin-film coating made of an electrically insulating material by using the conventional apparatuses shown in FIGS. 20 and 21, the insulating material adheres to the outer surface of the hearth 19 or anode 31 or the inner surface of the vacuum chamber 12 or the like, and in particular, the outer surface of the hearth 19 or anode 31 becomes electrically insulated. As a result, flow of electric current in the vacuum chamber 12 can not be maintained, so that the phenomenon that the electrodes are charged up at their various parts becomes marked as time elapses. Therefore, it becomes impossible to perform continuous and stable control for the plasma beam 22, causing a problem that the stability of coating formation is spoiled. Meanwhile, if such a phenomenon occurs, electrons incident on a portion where the flow of the electric current can not be maintained are reflected, and the reflection of electrons will be repeated until the electrons are electrically neutralized by combination with ions or until the electrons finally arrive at places where the electrons can be return electrically.
Further, since a magnetic field for controlling the plasma beam 22 is present in the vacuum chamber 12, the aforementioned reflected electrons, i.e., the motion of reflected electrons is restricted by this magnetic field. Accordingly, in order to form a thin-film coating made of insulation material continuously and stably by using the plasma gun 11, it is necessary to provide a proper return electrode for reflected-electrons at a position where an optimum state of magnetic field distribution is obtained and where an insulation coating is less likely to be deposited.
Unless the return path for reflected-electrons is urged to change, most of the reflected electrons tend to flow back in the path of the plasma beam 22 along this magnetic field under the effect of the magnetic field that controls the plasma beam 22. In other words, the plasma beam 22 and a beam by the reflected electrons reciprocate in the generally same path, whereas the reflected electrons is worse in convergency and smaller in acceleration voltage due to their reflection and scattering, compared with the electrons in the plasma beam 22.
Therefore, an electron beam incident on the return electrode for the reflected-electron needs to be separated from the plasma beam 22 delivered from the plasma gun 11. Without this separation, there would occur a plasma beam flowing directly into the return electrode for the reflected-electrons, so that the plasma beam reaching the deposition material side or the anode side would decrease in amount, resulting in a lowered efficiency of coating formation. Still more, direct discharge would further occur between the cathode 15 and the return electrode for the reflected-electron, making the discharge itself abnormal. The return electrode for the reflected-electrons is, preferably, provided at a position as far as possible from the portion in the vacuum chamber 12 where coating material is produced (position of the deposition material 20 in FIG. 20 or the gas outlet from the material gas supply tube in FIG. 21) from the viewpoint of preventing the deposition of the insulating material, and at a position as close as possible to the converging coil 18 from the viewpoint of reducing the device size.